A gas discharge device excited with high frequency (200 MHz to 10 GHz) electromagnetic fields for use, for example, in excitation of lasers for chemical processing of solid surfaces, is shown in U.S. Pat. No. 4,789,809 to Christensen (commonly assigned herewith), the disclosure of which is incorporated herein by reference. The device disclosed by Christensen uses a high-frequency electric field to excite a gaseous plasma between an electrically conducting electrode and a ground plane. The electrode is conductively or capacitively connected to a conductor that is positioned over the same ground plane to form a transmission line for delivery of high frequency power from the generator to the discharge. Impedance matching is accomplished by positioning two quarter wavelength slugs between the transmission line conductor and the ground plane to decrease the characteristic impedance in their vicinity. When applying low frequency voltages (0-50 MHz), a thin sheet of dielectric can be placed between the discharge electrode and the transmission line conductor.
This device has the disadvantage that the microstrip waveguide configuration used in the device is not capable of withstanding relatively high microwave power levels (on the order of a few hundred kilowatts or higher). The tuning slugs described in the aforementioned patent also are not capable of withstanding these higher microwave power levels without the formation of spurious discharges. There is also no provision for connecting the device to a hollow waveguide.